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Locate the switch and test it with an ohm meter to verify that it is malfunctioning. Before replacing the switch, shut off the power to the furnace. Remove the wires one by one, noting their original positions, then unscrew the control and remove it from the furnace. Place the new switch in the furnace and then screw the control back into the furnace, replacing the wires in their original configuration and ensuring all of the connections are secure.